This invention relates to electronic article surveillance systems and, in particular, to electronic article surveilance systems which utilize magnetic marker for surveillance purposes.
Eectronic article surveillance systems wherein magnet markers are attached to the articles under surveillance are well known in the art. In these systems, an alternating magnetic field is applied to an interrogation zone via an antenna system. If an article with a suitable magnetic marker then passes into the zone, the presence of the marker in the zone causes a perturbation in the field. The field in the zone is sensed by a receiver antenna, whose output contains this perturbation. A detector analyzes the signal from the receiver to assess whether a perturbation to the field has occurred and whether this perturbation is a result of a marker If so, an alarm is activated indicating passage of the marker through the zone.
In prior electronic article surveillance systems, the perturbations to the field created by a magnetic marker have been detected using frequency domain or time domain analysis techniques. Typically, when frequency domain techniques are employed, the frequency content of the received signal is investigated for harmonics of the fundamental (drive frequency) of the applied field These harmonics are generated as a result of the non-linear hysteresis characteristics of the magnetic marker. By comparing the relative amplitudes of the detected harmonics, an indication of the frequency spectrum of the signal is obtained. Using certain decision criteria this spectrum is then compared with the spectrum expected from a valid marker and a decision as to the presence of the marker is reached. The use of frequency domain analysis techniques is particularly desirable where noisy conditions may be expected, but the high Q filters required to isolate the generated harmonics make the system response time long.
In time domain analysis, the time domain pulse of the received signal is analyzed with respect to its pulse shape and its time displacement relative to the fundamental phase of the applied field. With this type of analysis, the shape of the signal is influenced significantly by the amplitude and group delay characteristics of the filtering used in the detection process and changes in the received signal due to variations in the applied drive field. Furthermore, because the analysis utilizes amplitude thresholds which must be set above the ambient noise level, the use of this procedure is most attractive where a high signal-to-noise ratio is present.
As can be appreciated, whether time domain or frequency domain analysis techniques are utilized in the surveillance system, it is essential that the detection process be able to discriminate between perturbations or changes in the field resulting from valid markers and those from non-marker sources. Failure of the system to provide the needed discrimination results in false alarms which significantly detract from the integrity and usefulness of the system.
The problem of reliably distinguishing between magnetic marker perturbations and extraneous non-marker perturbations is heightened in retail establishments particularly, supermarkets, where metallic fixtures, metal counters, metal shopping carts, noise generators (e.g., laser scanners, digital scales, credit card and bar code readers, conveyor belts, etc.) and the like proliferate. This equipment creates a harsh electronic environment and may cause perturbations in the detection system which either mask valid marker signals and/or appear as valid marker signals. The reliability of the system is, therefore, significantly compromised.
Present surveillance systems have been unable to completely solve this reliability problem and have suffered from other types of disadvantages. Thus, systems have been designed which are physically very large, causing flow through the interrogation zone to be impeded. Also, these systems have utilized relatively strong applied fields. As a result, the fields often extend beyond the interrogation zone, increasing the likelihood of perturbations from non-marker sources or from markers outside the interrogation zone. Furthermore, the overrange of these systems coupled with electronic noise significantly reduces system sensitivity. A relatively poor detection rate and an undesirable number of false alarms, thereby often result.
There has recently been developed a magnetic marker having attributes which can alleviate some of the aforementioned problems encountered in these prior systems. U.S. Pat. No. 4,660,025, entitled "Article Surveillance Magnetic Marker Having An Hysteresis Loop With Large Barkhausen Discontinuities", assigned to the same assignee hereof, discloses a magnetic marker of this type. The marker comprises a magnetic material having retained stress and a magnetic hysteresis loop which exhibits large Barkhausen discontinuities. As a result, when the marker is exposed to an applied magnetic field above a threshold value, the marker undergoes a regenerative reversal in its magnetic polarization.
Because this regenerative reversal can be made to occur at a relatively low threshold value, the applied field required for the marker can advantageously also be relatively low. Furthermore, the step function reversal exhibited by the magnetic polarization of the marker results in perturbations of the field which are rich in high harmonics, making detection easier and simpler.
The marker of the '025 patent is also advantageous in that it can be deactivated by a variety of practices as is disclosed in U.S. Pat. No. 4,686,516, entitled "Method, System and Appraratus for Article Surveillance", and also assigned to the same assignee hereof. Thus by alleviating the retained stress in the marker or by crystallizing a portion of the marker, the marker is easily deactivated so as to be able to pass through the interrogation zone without producing any alarm.
Another advance which is directed to overcoming the aforementioned problems of prior systems is disclosed in co-pending U.S. patent application Ser. No. 880,138 now U.S. Pat. No. 4,769,631 also assigned to the same assignee hereof and whose disclosure is incorporated herein by reference. The '138 application discloses a magnetic shield which is to be placed at the side edges of the interrogation zone and which is adapted to reduce the intensity of the magnetic field outside the zone. The shield is also adapted such that the perturbations to the field caused by the shield itself can be easily distinguished and blocked.
As disclosed in the '138 application, to achieve these characteristics, the magnetic material of the shield must have a sufficiently high resistivity, for a given permeability and frequency of the applied magnetic field, to provide a skin depth substantially in excess of the thickness of the shield. The magnetic material must also have a saturation flux density greater than the maximum flux density produced in the shield due to the positive and negative peak excursions of the applied field. Finally, the magnetic material must be such that it responds to the applied field at the peaks of the field with little or no response near the zero crossings.
As further discussed in the '138 application, shields made from ferrites and pressed powdered iron can provide the aforesaid characteristics and result in a ratio of peak front field (i.e., field inside the zone) to peak back field (i.e., field behind the shield) of at least ten to one. A particular ferrite which exhibits this 10 to 1 ratio and, in addition, a maximum response at the applied field peaks and a minimum response at the zero crossings is a ferrite identified commercially as Q5B, manufactured by TDK Corporation of Tokyo, Japan. Similar characteristics are found in pressed powdered iron, preferably unsintered, having about 99 percent iron content with about one percent trace elements including FeP, H.sub.2, C, Mn, S and perhaps other minute amounts of other elements.
Where the magnetic shield is a laminate of a plurality of thin sheets glued together and electrically isolated from one another, the skin depth of each such sheet is made substantially greater than the sheet thickness.
The need for minimum response of the shield near the zero crossings of the applied field is due to the fact that magnetic markers and, in particular, the markers of the '025 patent, have a maximum response at the zero crossings of the applied field and a minimum response at the field peaks. By selecting the shield magnetic material to have the reverse characteristic, perturbations caused by the shield can be easily detected and eliminated without interfering with detection of the marker perturbations.
The '138 patent application also teaches, in conjunction with the above discussed magnetic shield, an auxiliary shield of electrically conductive material. This shield is situated behind the magnetic shield and attenuates, through eddy current losses, perturbations from markers and external noise sources located outside the interrogation zone.
While the above magnetic marker and magnetic shield provide significant improvements to prior magnetic electronic article surveillance systems, there still exists a need for an overall system having attributes of substantial reliability, compactness and freedom from false alarms.
It is therefore a primary object of the present invention to provide an improved electronic article surveillance system and method.
It is a further object of the present invention to provide an improved magnetic electronic article surveillance system and method having enhanced control functions.
It is a further object of the present invention to provide an improved magnetic article surveillance system and method having enhanced detection practices.
It is yet a further object of the present invention to provide an improved magnetic electronic surveillance system and method having unique frequency domain and unique time domain detection practices.
It is still a further object of the present invention to provide an improved magnetic electronic article surveillance system and method which is responsive and self adjusting to changes in the surrounding environment including changes in noise.
It is a further object of the present invention to provide an improved magnetic electronic surveillance system having an antenna system with enhanced properties.
It is yet a further object of the present invention to provide an improved magnetic electronic surveillance system having an antenna pedestal with improved properties.
It is yet a further object of the present invention to provide an improved magnetic electronic surveillance system having unique comb bandpass filter means.
It is yet a further object of the present invention to provide an improved magnetic electronic surveillance system having a unique comb notch filter means.
It is a further object of the present invention to provide an improved magnetic electronic surveillance system as above described with the further addition of a magnetic marker as in the '025 patent and a shield as in the '138 application.